Bitcoin may be nowhere near the limit on coins, but the blockchain size is becoming an issue and there is disagreement on how to deal with that by expanding it, since it will obsolete older bitcoin clients.
It also isn't scalable at all, I've heard something like 3 transactions a second is about the limit it can handle. For a niche application like bitcoin is today that's not an issue, but those who wish it would become widespread would need to first completely re-architect the whole thing to address that. If someone piggybacked enough unrelated transactions into it, they could effectively DoS all bitcoin transactions. I'm actually surprised someone hasn't already done this.